Multiscale Analysis Of Bohai Ocean-effect Snowstorm

Ocean-effect snow occurs when cold air flows over a relatively warm water surface of Bohai, generating snow bands over and downwind of the sea and land, which is the most distinct in Shandong peninsula. It is called cold air snow or cold air snowstorm by local forecasters since it occurs under the settings of North-west cold advection in low troposphere.Basic ingredients and development mechanism is remarkable between Ocean-effect snow and other typological snow. Except this, theγ-scale narrow snowstorm band associate with these events, ranging from a few observation station, presents a difficult forecast challenge. The forecast of onset, location, intensity, movement, and persistence of these relatively shallow snow bands is difficult given the limitation of conventional observing system.To make an intensive study of Bohai Ocean-effect snowstorm (short for BOES), two BOES events including the unusual long snowstorms in December, 2005 and exceptional heavy snowfall in December,2008 are investigated with the data of synoptic analysis, diagnostic analysis, numerical simulation, radar wind retrieval. Multiscale analysis is laid heavy stress.The results are as following:1. Synoptic settings character of BOESIt shows that BOES occurs under the settings of North-west cold advection in low troposphere from eight snowstorm examples. In 500hPa chart, trough of low pressure is located east of Lake Baikal in middle latitude area. North-west wind blows under the layer of 850hPa in Bohai and Shandong peninsula. The isotherm is towards north-east and south-west, which is hardly vertical with isoheight, so extremely cold advection exists. In surface pressure field, huge cold high pressure controls Bohai sea and Shandong peninsula, and low pressure lies in Japan sea. Snows come about the cold front. If the cold front is secondary, snowfall can rise as the front is passing. The trough is trim by stern from surface to high layer.Snowstorms can last for several days if strong cold air affects Bohai frequently. In December, 2005, weather situation underwent double blocking anticyclone development, so low pressure maintained continuously in east of Lake Baikal in middle latitude area. The situation is favorable for cold air time after time to invade Bohai and more snowstorms. In contrast, once cold air only produce a snowstorm-day such as the example for December, 2008.2. Thermal character of BOESThere are several thermal characters for BOES as following:(1)Temperature near Chengshantou station at 850hPa is≤-12℃when BOES occurs.(2) BOES occurs in boundary layer. Thin convection is the important feature. cold air is heated and moistened as it passes over the warm Bohai , creating an unstable lapse rate. The vertical extent of the convection is limited by 850hPa.(3)Vertical thermal structure in Bohai and Shandong peninsula depends on intensity of cold air, which leads to the difference of snowfall in different phase. At the beginning, weak cold air only produce small unstable lapse rate and light snow. Snowfall can strength with cold air deepens.(4)Heavy snow falls near the ridge of equivalent potential temperature in low troposphere, remarkablely from 850hPa to 950hPa.3. Dynamic character of BOESFirstly, diagnostic analysis shows that it is convergent in low troposphere and divergent in high troposphere. Ascending motion is limited under 850hPa, no more than 700hPa.The feature is obvious different with rainstorm in summer and large range snowstorms. Secondly, as the air travels from the water surface over land, orographic lift and frictional convergence are involved. It can be the dominant factor in creating a snowstorm band near the north shore. Thirdly, the strongest snowfall occurs when the low pressure trough passes Bohai and Shandong peninsula.In this situation, a combination of ocean-effect and synoptic-scale processes contributes to snowstorm development.Fourthly, north-westerly blows vapour from Bohai sea to the land. Large moisture convergence is limited in 925hPa, which is different from other snow that south-westerly in high troposphere transfers vapour from Indian Ocean or the Bay of Bengal.4. Mesoscale character of BOESBy original radial velocity, reflectivity , EVAP retrieved wind fields based on Yantai Doppler weather radar data and simulation data of RAMS model, mesoscale character of BOES is revealed.(1)There is shear between north-east and north-west wind in low troposphere. Position of the shear affects the location of snowstorm. (2)Long term adverse wind region in Doppler radial velocity images is observed during the snowstorm. It means that mesoscale vertical circulation occurs at the same time.(3)There are four spatial types for BOES bands by radar radial velocity, including L snow bands, single snow bands, double snow bands and widespread snow bands.Types of snow bands tend to develop parallel to the prevailing wind flow in the low troposphere.5. Microphysics of BOESThe results of simulation data show precipitation microphysics in BOES. There are two kinds of microphysics at the course of BOES. One is called"seed-feed"mechanism.Middle-level clouds can produce ice crystals that"seed"the lower ocean effect clouds below. In this process, ice crystals generate in the higher clouds fall into the lower ocean-effect clouds, allowing further snowflake growth by deposition and aggregation. This can enhance the precipitation production and ocean-effect snowfall. This process occurs before the low pressure trough at 500hPa passes Bohai sea. The other favorable factor is temperature. Temperature within the cloud layer plays an important role in enhancing the precipitation production process and determining ice crystal structure in ocean-effect snow events. Temperature is lower than -10°C in the process, so it is favorable for growth of dendrites.6. Terrain lift on BOESTerrain plays a major role on BOES. In this paper, initial analysis on the eastwestly hills in Shandong peninsula is conducted with the observational snowfall, automatic observation wind field and data of simulation. It is found that northwest wind converts to west-northwest wind and wind speed decreases when the wind blows from Bohai to north shores in Shandong peninsula. It leads to convergence and aloft motion in the north of the hills, while the opposite situation in the south of the hills. That is the main reason why snowfall is much heavier in the north of hills.7. Multiscale conceptual model of BOESMultiscale conceptual model of BOES can be summarized by the results in this paper. The favorable synoptic setting is critically important for extremely cold air. When cold air flows Bohai sea from northwest land, warm sea surface transfer heat and moisture to the upper cold air so that unstable lapse rate forms in low troposphere. Orographic Lift and frictional Convergence produces mesoscale shear, which triggers to release the unstable energy. Thus Bohai ocean-effect snow occurs. The heaviest snowfall produces while the low pressure trough passes Bohai. In the process, there are three actions combine. Firstly, low pressure trough leads strong cold air to burst southwards to strengthen the BOES. Secondly, positive vorticity advection enhances upward motion. Thirdly, microphysics mechanism"seed-feed"allows further snowflake growth. In this situation, the combination of ocean-effect , microphysics and synoptic-scale processes contributes to snow cloud development. As a result, large scale, mesoscale, cloud scale and ocean-effect combine action on BOES.8. Forecast indexes of BOESBased on the results, several indexes of BOES for the forecast operation are suggested.(1) BOES occurs mainly in the middle and late November, early and middle December, especially in December. In the climate background, forecasters are suggested to consider the probability of BOES when forecasting strong cold air can pass Bohai.(2)It is the primary problem that if synoptic setting is favorable to strong cold air to burst southwards. Here is the representative synoptic setting of BOES. Vortex is found from North-east China to Japan sea in 500hPa.And there is cold advection in troposphere. At surface synoptic chart, strong cold anticyclone controls the mid and east China. These clues mean strong cold air can come to Bohai. If north-west cold advection is forecasted in Bohai and Shandong peninsula under the layer of 850hPa and cold front has passed Shandong peninsula to the east sea, heavy ocean-effect snow may produce.(3)Temperature is lower than -12℃in the east of Shandong peninsula at 850 is the necessary factor for BOES in December.(4)The next attention is horizontal wind in low troposphere. Shear of north-east and north-west wind affects the location of BOES. In the forecast operation, forecasters can analyze Doppler radar radial velocity, automatic station wind and products of mesoscale models.(5)Phase of heavy snowfall can be estimated by the low pressure trough at 500hPa.Snowfall increases when the trough passes Bohai and Shandong peninsula. Deep and resupinate trough is more favorable to heavy BOES. Development of potential vorticity is a good index.(6)Persistent BOES depends on the block anticyclone. In the course of formation, maintenance, development and reduction of block anticyclone, strong cold air can invade Bohai and Shandong peninsula frequently to produce BOES many times.